In a motor vehicle, it is known to use what are termed ‘detection’ sensors to determine the position of a drive shaft or else its rotational speed. Such a shaft may be for example a crankshaft, a camshaft or a transmission shaft of a motor vehicle.
The sensor is installed facing a target that is fixed coaxially on the drive shaft. This target takes the form, as is known, of a crown made of a ferromagnetic material and including a sequence of teeth and cavities.
As is also known, the sensor comprises an electric power supply connector by way of which it is supplied with DC voltage, for example 5 V, so as to operate in the vehicle, an electric output connector and a ground connector.
In order to determine the position or the speed of the shaft, the sensor includes an integrated circuit configured to measure the magnetic field variations generated by the passing of the teeth and cavities of the target in front of the sensor.
The sensor thus generates a digital output signal representing the passing of the teeth and of the cavities characterizing the position or the rotational speed of the shaft. This signal is supplied, via the output connector, to a computer of the vehicle, which analyzes said signal in order to determine the position or the rotational speed of the shaft.
In order to allow such a sensor to meet electromagnetic compatibility standards, it is known to attenuate the power supply signal of the sensor by installing a filtering circuit in the sensor, between the connectors and the integrated circuit.
Moreover, it is known to carry out tests on this type of sensor using a test device that communicates information with the sensor via a test protocol.
The tests performed on the sensor may consist in collecting samples of the output signal from the sensor representing periodic variations in the magnetic field that are generated by the rotation of the target in front of the sensor at a given speed, for example several samples per period of the signal.
To send test commands to the sensor, the test device generates a test signal including test information, which signal is sent on the power supply line of the sensor. Thus, when the sensor is used in the vehicle during operation, it is supplied with a DC voltage power supply signal, for example with a value of the order of 5 V, but when the sensor has to be tested by the test device, the latter delivers, on the power supply line of the sensor, a pulsed voltage test signal defined by a test protocol and representative of the test information.
As illustrated in FIG. 1, this test signal (denoted ‘VG’ in the figure) may consist in exchanging binary information of 0 or 1 type, for example coded in terms of pulse width in the form of high states EH with a width of ⅓ to represent the binary value 1 alternating with low states EB with a width of ⅔ to represent the binary value 0. In FIG. 1, the high states EH have a value of the order of 17 V and the low states EB have a value of the order of 6 V, so as easily to be able to distinguish them from one another. However, as these values are able to vary, the sensor detects a high state EH of the test signal when the latter exceeds a predetermined high-state detection threshold SDH (of 14 V in FIG. 1). Likewise, a low state EB may be detected when the value of the test signal is lower than a predetermined low-state detection threshold SDB (of 9 V in FIG. 1).
When it is desired to test the sensor at high rotational speeds of the shaft, for example higher than 500 RPM, it is necessary to deliver a test signal the frequency of which is high enough to allow collection of several samples per period of the signal representative of the variations in magnetic field, for example at least 20 kHz.
However, at such frequencies, the test signal may be attenuated significantly by the filtering circuit (the filtered test signal being denoted ‘VFilt’ in FIG. 1), such that it may not cross the predetermined high-state detection threshold SDH. In this case, the sensor is not able to determine whether binary test information is contained in the test signal, thereby preventing the test device from collecting the samples, and therefore constituting a major drawback.